Drain Bolt Mounting Structure

ABSTRACT

A drain bolt mounting structure comprises a bottomed hole formed on an outer surface of an oil pan to extend continuously with a drain hole of the oil pan, a female thread formed on an inner peripheral surface of the drain hole; a drain bolt including a shaft portion having a male thread threadingly engageable with the female thread of the drain hole and a head portion formed at one end portion of the shaft portion; a press-in member formed of a second metal containing aluminum which has a higher standard electrode potential than a first metal forming the oil pan, the press-in member being configured to be pressed into the bottomed hole; and a crushing member configured to be crushed between the press-in member and the head portion of the drain bolt.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent Application No. 2009-297894 filed on Dec. 28, 2009 which is hereby incorporated by reference in its entirety for all purposes.

BACKGROUND ART

1. Field of the Invention

The present invention relates to a drain bolt mounting structure for sealably closing a drain hole provided in an oil pan formed of metal containing magnesium, in a fluid-tight state.

2. Description of the Related Art

An engine mounted in a vehicle such as a motorcycle includes a cylinder head, a cylinder block, a crankcase, an oil pan, etc. The crankcase accommodates a crankshaft and an oil pump. The oil pan is provided with a drain hole at a bottom portion thereof. The drain hole is sealably closed by a drain bolt or the like in a fluid-tight state. Upon the engine running, the oil pump is actuated by the rotation of the crankshaft, and oil reserved in the oil pan is pumped by the oil pump to be fed to driving members and components to be cooled.

The drain bolt mounting structure is required to provide high fluid-tightness to prevent leakage of oil to an outside of the oil pan and to prevent the drain bolt from coming loose. In a case where an oil pan formed of magnesium or magnesium alloy is used to provide, for example, a lightweight engine, the oil pan is required to be subjected to an electric corrosion resistance treatment, because magnesium has a very low standard electrode potential and is likely to be electrically corroded due to contact with other metal (dissimilar metal contact).

Japanese Patent No. 3299526 is directed to solving the above mentioned problem, and discloses that a high fluid-tight state is attained using annular seal projections (16) and (23), a washer (11) prevents a bolt (12) from coming loose, and an anti-corrosion plate (14) prevents electric corrosion of an oil pan (1) formed of magnesium alloy. The reference characters in parentheses are recited in the above patent and used hereinafter.

In the drain bolt mounting structure disclosed in the above Japanese Patent, since the anti-corrosion plate (14) is sandwiched between a side wall (2) of the oil pan (1) and the washer (11), the anti-corrosion plate (14) is completely separated from the oil pan (1) when the bolt (drain bolt) (12) is removed to, for example, change the oil. To re-mount the bolt (12) to the oil pan (1), it is necessary to re-position the anti-corrosion plate (14). Thus, the bolt (12) is not easily and efficiently mounted.

Since the annular seal projections (16) are formed at the end surface of the anti-corrosion plate (14) and are engaged with the side wall (2), the side wall (2) will be damaged due to repeated mounting and removal of the bolt (12), and a sealed state between the anti-corrosion plate (14) and the side wall (2) will be deteriorated, thereby causing the oil to leak through a clearance between the anti-corrosion plate (14) and the side wall (2).

If water such as rain water enters a clearance between the anti-corrosion plate (14) and the side wall (2) because their sealed state becomes degraded, the electric corrosion of the side wall (2) formed of magnesium alloy will progress, thereby causing the bolt (12) to come loose.

SUMMARY OF THE INVENTION

The present invention addresses the above described conditions, and an object of the present invention is to provide a drain bolt mounting structure which allows the drain bolt to be mounted more easily and efficiently, can surely attain a highly sealed state, and can prevent the drain bolt from coming loose due to electric corrosion.

To solve the above mentioned problem, according to the present invention, there is provided a drain bolt mounting structure for sealably closing a drain hole provided on an oil pan formed of a first metal containing magnesium, in a fluid-tight state, the drain bolt mounting structure comprising a bottomed hole formed on an outer surface of the oil pan to extend continuously with the drain hole, an end portion of the drain hole being open in a part of a bottom surface of the bottomed hole; a female thread formed on an inner peripheral surface of the drain hole; a drain bolt including a shaft portion having a male thread threadingly engageable with the female thread of the drain hole and a head portion formed at one end portion of the shaft portion to protrude in a radial direction of the shaft portion; a press-in member formed of a second metal containing aluminum which has a higher standard electrode potential than the first metal and having a first hole into which the shaft portion of the drain bolt is inserted, the press-in member being configured to be pressed into the bottomed hole such that an outer peripheral surface of the press-in member is tightly fitted to an inner peripheral surface of the bottomed hole; and a crushing member having a second hole into which the shaft portion of the drain bolt is inserted, the crushing member being configured to be crushed between the press-in member and the head portion of the drain bolt.

In such a configuration, since the press-in member is pressed into the bottomed hole formed on the outer surface of the oil pan, it is possible to prevent the press-in member from coming off from the bottomed hole when the drain bolt is detached from the oil pan. When the drain bolt is re-mounted to the oil pan, a messy operation, for example, re-positioning of the press-in member and pressing of the press-in member into the bottomed hole, is not necessary. Thus, the drain bolt is mounted more easily and more efficiently.

In addition, since the press-in member is pressed into the bottomed hole such that the outer peripheral surface of the press-in member is tightly fitted to the inner peripheral surface of the bottomed hole, a highly sealed state is surely attained between the inner peripheral surface of the bottomed hole and the outer peripheral surface of the press-in member. Since the crushing member is crushed and deformed between the press-in member and the head portion of the drain bolt by tightening the drain bolt, the crushing member and the press-in member are tightly fitted to each other and the crushing member and the head portion of the drain bolt are tightly fitted to each other. Thus, a highly sealed state is attainable between the crushing member and the press-in member and between the crushing member and the head portion of the drain bolt. This makes it possible to prevent the oil from leaking from the oil pan to an outside of the oil pan through the drain hole.

Since the bottom surface of the bottomed hole which is a part of the oil pan is formed of the first metal containing magnesium and the press-in member configured to contact the bottom surface of the bottomed hole is formed of the second metal containing aluminum which has a higher standard electrode potential than the first metal, the electric corrosion of the bottom surface of the bottomed hole can be suppressed by reducing a standard electrode potential difference between the bottom surface of the bottomed hole and the press-in member. Since a highly sealed state is surely attained between the inner peripheral surface of the bottomed hole and the outer peripheral surface of the press-in member as described above, water such as rain water can be prevented from entering the clearance between the bottom surface of the bottomed hole and the press-in member, which makes it possible to prevent the electric corrosion of the bottom surface of the bottomed hole due to the water. Therefore, degradation and deformation of the bottom surface of the bottomed hole due to the electric corrosion, which will cause the drain bolt to come loose, is effectively prevented.

At least a surface of the crushing member which is opposite to the press-in member may be formed of a third metal containing aluminum which has a lower hardness than a surface of the press-in member which is opposite to the crushing member.

In such a configuration, since at least the surface of the crushing member which is opposite to the press-in member has a lower hardness than the surface of the press-in member which is opposite to the surface of the crushing member, the crushing member can be preferentially crushed while suppressing damage to the press-in member when tightening the drain bolt. By changing only the crushing member damaged by the crushing into a new one, a product quality can be maintained for a longer period of time. Since the crushing member is formed of the third metal containing inexpensive aluminum, the crushing member can be changed into a new one at a low cost.

At least the surface of the press-in member which is opposite to the crushing member may be a surface subjected to a hardening treatment.

The surface of the press-in member is hardened by a hardening treatment. As used herein, the hardening treatment refers to a treatment for secondarily hardening the surface of the press-in member formed of the second metal containing aluminum, and may be a film coating treatment (plating, etc.) for forming a coating film of a hard metal on the surface, an oxidation treatment (alumite treatment, etc.) for forming an oxide coating film on the surface, a forging treatment for hammering the surface to harden the same, etc.

An O-ring may be provided between the bottom surface of the bottomed hole and the press-in member and around the shaft portion of the drain bolt.

In such a configuration, leakage of oil from the oil pan, and entry of water such as rain water into the oil pan can be surely prevented because of a sealing function of the O-ring.

The above and further objects and features of the invention will more fully be apparent from the following detailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a left side view showing a construction of an entire motorcycle including an oil pan having a drain bolt mounting structure according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a construction of the oil pan having the drain bolt mounting structure according to the embodiment and a crankcase.

FIG. 3 is a cross-sectional view showing the drain bolt mounting structure according to the embodiment.

FIG. 4 is an exploded perspective view showing the drain bolt mounting structure according to the embodiment.

FIG. 5 is a cross-sectional view of the drain bolt mounting structure according to the embodiment, from which a drain bolt and a crushing member are removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The stated directions are referenced from the perspective of a driver straddling a motorcycle, unless otherwise explicitly noted.

[Construction of Motorcycle]

FIG. 1 is a left side view showing a construction of an entire motorcycle 14 including an oil pan 12 having a drain bolt mounting structure 10 according to the embodiment.

Referring now to FIG. 1, the motorcycle 14 includes a main frame member 16, a head pipe 18 provided at the front portion of the main frame member 16 and a pair of right and left pivot frame members 20 provided at the rear portion of the main frame member 16. A steering shaft (not shown) is rotatably inserted into the head pipe 18. A front fork 22 and a steering handle 24 are attached to the steering shaft. A pair of right and left swing arms 26 is attached to the pivot frame members 20, respectively. A front wheel 28 is mounted to the lower end portion of the front fork 22. A rear wheel 30 is mounted to the rear end portions of the swing arms 26. A fuel tank 32 and a seat 34 are arranged at the upper portion of the main frame member 16 such that the fuel tank 32 is disposed forward relative to the seat 34. An engine E is mounted at the center portion in a space defined by the main frame member 16 below the fuel tank 32.

The engine E includes a cylinder head 36, a cylinder block 38, a crankcase 40, and an oil pan 12. Although not shown, a combustion chamber is formed inside the cylinder head 36. A fuel is fed from the fuel tank 32 to the combustion chamber and combusted therein. A piston driven to reciprocate by the combustion of the fuel is accommodated in the cylinder block 38. A crankshaft driven to rotate by the reciprocating piston is accommodated in the crankcase 40.

FIG. 2 is a cross-sectional view showing a construction of the crankcase 40 and the oil pan 12. Referring to FIG. 2, the crankcase 40 accommodates the crankshaft (not shown), an oil pump 44 driven by the rotation of the crankshaft and an oil passage 46 through which oil pumped by the oil pump 44 flows. The oil pan 12 reserves the oil. The oil pan 12 is provided with an oil guide member 48 inside thereof. The oil guide member 48 is a pipe member used to deliver the oil from inside the oil pan 12 to the oil pump 44. An upper end portion 48 a of the oil guide member 48 is coupled to a suction port 44 a of the oil pan 12. A lower end portion 48 b of the oil guide member 48 is disposed at the bottom portion of the oil pan 12. An oil filter 50 is attached to the lower end portion 48 b to remove foreign matters and others in the oil. A drain hole 52 is provided at the bottom portion of the oil pan 12. The drain bolt mounting structure 10 sealably closes the drain hole 52 in a fluid-tight state.

At least the oil pan 12 is formed of metal containing magnesium (hereinafter referred to as first metal) to provide, for example, the lightweight engine E. The first metal is desirably composed of magnesium as a major component for the purpose of reducing the weight. However, the entire first metal need not be magnesium but may be an alloy of magnesium and another metal (aluminum, zinc, zirconium, manganese, thorium, etc.). Since magnesium has a very low standard electrode potential, the oil pan 12 formed of the first metal containing magnesium is easily electrically corroded due to contact (dissimilar metal contact) with another metal.

[Construction of Drain Bolt Mounting Structure]

FIG. 3 is a cross-sectional view showing the drain bolt mounting structure 10 according to the embodiment. FIG. 4 is an exploded perspective view showing the drain bolt mounting structure 10 according to the embodiment.

As shown in FIGS. 3 and 4, the drain bolt mounting structure 10 is configured to sealably close the drain hole 52 provided at the oil pan 12 formed of the first metal in a fluid-tight state while preventing the electric corrosion of the oil pan 12, and includes a bottomed hole 60 formed on an outer surface 12 a of the oil pan 12, a female thread 62 formed on the inner peripheral surface of the drain hole 52, a drain bolt 64, a press-in member 66, a crushing member 68, and an O-ring 70.

The bottomed hole 60 is a circular hole formed on the outer surface 12 a of the oil pan 12 continuously with the drain hole 52. An end portion 52 a of the drain hole 52 opens in a portion (in this embodiment, center portion) of a bottom surface 60 a of the bottomed hole 60. The bottom surface 60 a of the bottomed hole 60 is smooth and flat to allow the bottom surface 60 a to be in surface contact with a surface 66 a of the press-in member 66 which is closer to the oil pan 12. An inner peripheral surface 60 b of the bottomed hole 60 is smooth and curved to allow the inner peripheral surface 60 b to be in surface contact with an outer peripheral surface 66 c of the press-in member 66. The bottomed hole 60 has a guide surface 60 c which is formed by chamfering in a taper shape, at the inner periphery of the end portion of an opening portion thereof. In this embodiment, since the end portion 52 a of the drain hole 52 opens in the center portion of the bottom surface 60 a of the bottomed hole 60, a space in which the O-ring 70 is disposed is formed around the end portion 52 a inside the bottomed hole 60.

The shape of the bottomed hole 60 is not particularly limited but may be an oval or a polygon (rectangle, hexagon) as well as the circle illustrated in this embodiment.

The female thread 62 serves as a joint portion to which a male thread 72 of the drain bolt 64 is joined. As the diameter or length of the female thread 62 increases, an area of a portion of the female thread 62 which is joined to the male thread 72 increases, which increases a joint force. However, since the diameter of the female thread 62 affects the diameter of the drain hole 52, i.e., an oil drainage amount, the size of the diameter of the female thread 62 is limited. As a solution to this, in this embodiment, as shown in FIG. 3, the wall thickness of a part of the bottom portion of the oil pan 12 is increased to form a thick portion 12 b, and the drain hole 52 and the female thread 62 are formed at the thick portion 12 b to increase the length of the female thread 62. In this way, the joint force is increased.

The drain bolt 64 includes a shaft portion 74 having on the outer peripheral surface thereof the male thread 72 which is threadingly engageable with the female thread 62, and a head portion 76 provided at one end portion 74 a (base end portion) of the shaft portion 74 to protrude in the radial direction of the shaft portion 74. The length of the shaft portion 74 is set longer than a sum of the length of the female thread 62, the thickness of the press-in member 66 and the thickness of the crushing member 68. This enables the male thread 72 to be securely joined to (engaged with) the whole length of the female thread 62. The head portion 76 includes an engagement portion 78 of a substantially hexagonal cylinder shape into which a tool such as a screw wrench is engaged and a circular seat portion 80 provided at the end portion of the engagement portion 78 which is closer to the shaft portion 74. An end surface 80 a of the seat portion 80 which is closer to the shaft portion 74 is smooth and flat to allow the end surface 80 a to be in surface contact with the surface 68 b of the crushing member 68 which is closer to the head portion 76 (seat portion 80).

The material of the drain bolt 64 is not particularly limited, but may be metal containing iron, aluminum, or brass.

The press-in member 66 is a member pressed into the bottomed hole 60 such that the outer peripheral surface 66 c thereof is tightly fitted to the inner peripheral surface 60 b of the bottomed hole 60. The press-in member 66 is, in this embodiment, a plate member, and is formed of metal (hereinafter referred to as second metal material) containing aluminum having a higher standard electrode potential than the first metal forming the oil pan 12. The press-in member 66 is provided with a circular through-hole 82 which is a first hole into which the shaft portion 74 of the drain bolt 64 is inserted. The press-in member 66 has a guide surface 66 e, formed by chamfering in a taper shape, at the outer periphery of the end portion of the press-in member 66 which is closer to the oil pan 12. An annular groove 84 is formed on the surface 66 a of the press-in member 66 which is opposite to the oil pan 12 to surround the through-hole 82. The O-ring 70 is accommodated in the annular groove 84. The depth of the groove 84 is set smaller than the diameter of the O-ring 70 in cross-section. With the O-ring 70 accommodated into the groove 84, the O-ring 70 is disposed between the bottom surface 60 a of the bottomed hole 60 and the press-in member 66 (i.e., bottom surface 84 a of the groove 84), around the shaft portion 74. In addition, a part of the O-ring 70 protrudes from the opening of the groove 84 to contact the bottom surface 60 a of the bottomed hole 60.

The outer peripheral surface 66 c of the press-in member 66 has a smooth surface (in this embodiment, curved surface) having the same shape (in this embodiment, circular shape) as that of the inner peripheral surface 60 b of the bottomed hole 60 to allow the outer peripheral surface 66 c to be tightly fitted to the inner peripheral surface 60 b of the bottomed hole 60. An inner peripheral surface 66 d (inner peripheral surface of the through-hole 82) of the press-in member 66 has a circular shape having an inner diameter which is slightly larger than the diameter of the shaft portion 74. The surface 66 a of the press-in member 66 which is opposite to the oil pan 12 is smooth and flat to allow the surface 66 a to be in surface contact with the bottom surface 60 a of the bottomed hole 60. The surface 66 b of the press-in member 66 which is opposite to the crushing member 68 is smooth and flat to allow the surface 66 b to be in surface contact with the surface 68 a of the crushing member 68 which is opposite to the press-in member 66.

The second metal forming the press-in member 66 desirably contains aluminum as a major component because aluminum is lightweight and inexpensive, has a small standard electrode potential difference with respect to the first metal (including magnesium), and causes the oil pan 12 to be electrically corroded less easily. However, the second metal need not be aluminum, but may be alloy containing aluminum and other metal (copper, zinc, magnesium, etc.).

At least the surface 66 b of the press-in member 66 which is opposite to the crushing member 68 desirably has a higher hardness than the surface 68 a of the crushing member 68 which is opposite to the press-in member 66 to allow the crushing member 68 to be preferentially crushed while suppressing damage to the press-in member 66. In order to increase the hardness of the surface 66 b of the press-in member 66, for example, the press-in member 66 may be formed of aluminum alloy (second metal) containing a hard metal to increase the hardness of the entire press-in member 66. Or, the entire surface of the press-in member 66 may be subjected to a hardening treatment to increase the hardness of the entire surface. Or, only the surface 66 b of the press-in member 66 may be subjected to the hardening treatment to harden only the surface 66 b. As the hardening treatment, for example, a film coating treatment (plating, etc.) for forming a coating film of a hard metal on the surface 66 b, an oxidation treatment (alumite treatment, etc.) for forming an oxide coating film on the surface 66 b, a forging treatment for hammering the surface 66 b, etc., may be selected and suitably used.

The crushing member 68 is crushed between the press-in member 66 and the head portion 76 of the drain bolt 64. In this embodiment, the crushing member 68 is a plate member. At least the surface 68 a of the crushing member 68 which is opposite to the press-in member 66 is formed of a metal (hereinafter referred to as a third metal) containing aluminum having a lower hardness than the surface 66 b of the press-in member 66 which is opposite to the crushing member 68. The crushing member 68 has at a center portion thereof a circular through-hole 86 which is a second hole into which the shaft portion 74 of the drain bolt 64 is inserted. The outer peripheral surface 68 c of the crushing member 68 has a circular shape having a smaller outer diameter than the outer peripheral surface 66 c of the press-in member 66 to allow the crushing member 68 to be crushed evenly between the press-in member 66 and the head portion 76 (seat portion 80). The inner peripheral surface (inner peripheral surface of the through-hole 86) 68 d of the crushing member 68 has a circular shape having an inner diameter slightly larger than the diameter of the shaft portion 74 to allow the inner peripheral surface 68 d to contact evenly the outer peripheral surface of the shaft portion 74 when the crushing member 68 is crushed. The surface 68 a of the crushing member 68 which is opposite to the press-in member 66 is smooth and flat to allow the surface 68 a to be in surface contact with the surface 66 b of the press-in member 66. The surface 68 b of the crushing member 68 which is closer to the head portion 76 (seat portion 80) is smooth and flat to allow the surface 68 b to be in surface contact with the end surface 80 a of the seat portion 80.

The third metal forming the crushing member 68 desirably contains aluminum as a major component because aluminum is lightweight and inexpensive, and has a hardness low enough to be crushed by a force for tightening the drain bolt 64. However, the entire third metal need not be made of aluminum, but may be made of alloy containing aluminum and other metal (copper, zinc, magnesium, etc.). The entire crushing member 68 may have a lower hardness than the surface 66 b of the press-in member 66, or only the surface 68 a of the crushing member 68 may have a lower hardness than the surface 66 b of the press-in member 66.

The O-ring 70 is compressed between the bottom surface 60 a of the bottomed hole 60 and the press-in member 66 (bottom surface 84 a of the groove 84), around the shaft portion 74, to seal them in a fluid-tight state. In the sealed state, the oil is prevented from leaking to outside of the O-ring 70 and water such as rain water is prevented from entering inside of the O-ring 70.

Although in this embodiment, the annular groove 84 is formed on the surface 66 a of the press-in member 66 which is opposite to the oil pan 12 to accommodate the O-ring 70 therein, an annular groove (not shown) may be formed on the bottom surface 60 a of the bottomed hole 60 formed on the oil pan 12 to accommodate the O-ring 70 therein. In a further alternative, an annular groove may be formed on the surface 66 a of the press-in member 66 and an annular groove may be formed on the bottom surface 60 a of the bottomed hole 60 such that these grooves are opposite to each other, and the O-ring 70 may be accommodated in an annular space formed by these annular grooves.

[Method of Manufacturing Drain Bolt Mounting Structure and Method of Using Drain Bolt Mounting Structure]

As shown in FIG. 4, when manufacturing the drain bolt mounting structure 10, initially, the thick portion 12 b, the drain hole 52, the bottomed hole 60 and the female thread 62 are formed integrally at the bottom portion of the oil pan 12 concurrently with manufacturing of the oil pan 12. Then, the O-ring 70 is attached to the groove 84 of the press-in member 66. The press-in member 66 is pressed into the bottomed hole 60. The outer diameter of the press-in member 66 is set slightly larger than the inner diameter of the bottomed hole 60 before the press-in member 66 is pressed into the bottomed hole 60. When the press-in member 66 is pressed into the bottomed hole 60, at least either one of the outer peripheral portion of the press-in member 66 and the inner peripheral portion of the bottomed hole 60 is crushed by a press-in force. After the press-in member 66 is pressed into the bottomed hole 60, the outer peripheral surface 66 c of the press-in member 66 is tightly fitted to the inner peripheral surface 60 b of the bottomed hole 60. In this press-in step, a great force for pressing the press-in member 66 into the bottomed hole 60 is required. Since the guide surface 60 c is formed on the inner peripheral edge of the end portion of the opening portion of the bottomed hole 60 and the guide surface 66 e is formed on the outer peripheral edge of the end portion of the press-in member 66 which is closer to the oil pan 12, the press-in step is easily carried out by butting these guide surfaces 60 c and 66 e with each other.

When the press-in of the press-in member 66 into the bottomed hole 60 is completed, the crushing member 68 is disposed on the surface 66 b of the press-in member 66, the shaft portion 74 of the drain bolt 64 is inserted into the through-hole 86 of the crushing member 68 and the through-hole 82 of the press-in member 66, and the male thread 72 formed on the shaft portion 74 is threaded into the female thread 62. Thereupon, the head portion 76 (end surface 80 a of the seat portion 80) of the drain bolt 64 is brought into contact with the surface 68 b of the crushing member 68, and the crushing member 68 is sandwiched between the press-in member 66 and the head portion 76. Under this condition, the crushing member 68 is not crushed yet, and there are small clearances between the crushing member 68 and the press-in member 66, between the crushing member 68 and the head portion 76, and between the crushing member 68 and the shaft portion 74. By further tightening the drain bolt 64, the crushing member 68 is crushed and deformed, thereby sealing the clearances in a fluid-tight state.

In accordance with the drain bolt mounting structure 10 manufactured as described above, since the outer peripheral surface 66 c of the press-in member 66 and the inner peripheral surface 60 b of the bottomed hole 60 are tightly fitted to each other, a highly sealed state is surely attained between the press-in member 66 and the bottomed hole 60. In addition, since the crushing member 68 seals the clearance and the O-ring 70 is disposed between the bottom surface 60 a of the bottomed hole 60 and the press-in member 66, the leakage of the oil can be surely prevented.

Since a highly sealed state is surely attained between the outer peripheral surface 66 c of the press-in member 66 and the inner peripheral surface 60 b of the bottomed hole 60, the water such as rain water is effectively prevented from entering the clearance between the bottom surface 60 a of the bottomed hole 60 and the press-in member 66. This prevents the bottom surface 60 a of the bottomed hole 60 from being electrically corroded due to the water. In addition, because of the fact that a standard electrode potential difference between the first metal forming the bottom surface 60 a of the bottomed hole 60 and the second metal forming the press-in member 66 is small, degradation or deformation of the bottom surface 60 a of the bottomed hole 60, due to the electric corrosion, is effectively suppressed, and as a result, the drain bolt 64 is prevented from coming loose due to the electric corrosion.

FIG. 5 is a cross-sectional view of the drain bolt mounting structure 10 according to the embodiment, from which the drain bolt 64 and the crushing member 68 are removed. Referring to FIG. 5, when the drain hole 52 is opened to change the oil, etc., the drain bolt 64 and the crushing member 68 are removed. In this embodiment, since the press-in member 66 is pressed into and secured to the bottomed hole 60, the press-in member 66 and the O-ring 70 do not come off from the bottomed hole 60 by removing the drain bolt 64. Therefore, a messy operation, for example, re-positioning of the press-in member 66 and the O-ring 70 and pressing of the press-in member 66 into the bottomed hole 60, need not be performed when the drain bolt 64 is re-mounted. To effectively carry this out, a force applied to secure the press-in member 66 to the inside of the bottomed hole 60, i.e., a frictional force generated between the outer peripheral surface 66 c of the press-in member 66 and the inner peripheral surface 60 b of the bottomed hole 60 is required to be larger than a resilient force generated by the O-ring 70. In this embodiment, to satisfy this condition, the size of the O-ring 70, a compression rate of the O-ring 70, the outer diameter of the press-in member 66 before being pressed into the bottomed hole 60, and a press-in depth of the press-in member 66, etc., are designed.

Since at least the surface 68 a of the crushing member 68 which is opposite to the press-in member 66 has a lower hardness than the surface 66 b of the press-in member 66, which is opposite to the surface 68 a in this embodiment, it is possible to crush the crushing member 68 preferentially while suppressing damage to the press-in member 66 when the drain bolt 64 is tightened. When the drain bolt 64 is re-mounted, only the crushing member 68 which has been damaged by the crushing is changed into a new one. In this way, a product quality can be maintained for a long period of time. Furthermore, since the crushing member 68 is formed of the third metal containing inexpensive aluminum, the crushing member 68 can be changed into a new one at a low cost.

As should be readily appreciated from the above, the drain bolt mounting structure 10 of the present invention allows a sealed state to be improved, the drain bolt 64 to be prevented from coming loose, the constituents and members to be easily and efficiently mounted, and a product quality to be maintained. The drain bolt mounting structure 10 is widely applied to oil pans which achieve such advantages.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments are therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims. 

1. A drain bolt mounting structure for sealably closing a drain hole provided on an oil pan formed of a first metal containing magnesium, in a fluid-tight state, the drain bolt mounting structure comprising: a bottomed hole formed on an outer surface of the oil pan to extend continuously with the drain hole, an end portion of the drain hole being open in a part of a bottom surface of the bottomed hole; a female thread formed on an inner peripheral surface of the drain hole; a drain bolt including a shaft portion having a male thread threadingly engageable with the female thread of the drain hole and a head portion formed at one end portion of the shaft portion to protrude in a radial direction of the shaft portion; a press-in member formed of a second metal containing aluminum which has a higher standard electrode potential than the first metal and having a first hole into which the shaft portion of the drain bolt is inserted, the press-in member being configured to be pressed into the bottomed hole such that an outer peripheral surface of the press-in member is tightly fitted to an inner peripheral surface of the bottomed hole; and a crushing member having a second hole into which the shaft portion of the drain bolt is inserted, the crushing member being configured to be crushed between the press-in member and the head portion of the drain bolt.
 2. The drain bolt mounting structure according to claim 1, wherein at least a surface of the crushing member which is opposite to the press-in member is formed of a third metal containing aluminum which has a lower hardness than a surface of the press-in member which is opposite to the crushing member.
 3. The drain bolt mounting structure according to claim 2, wherein at least the surface of the press-in member which is opposite to the crushing member is a surface subjected to a hardening treatment.
 4. The drain bolt mounting structure according to claim 1, wherein an O-ring is provided between the bottom surface of the bottomed hole and the press-in member and around the shaft portion. 